Chapter 19
The kinetic theory of gases
In this chapter we will introduce the kinetic theory of gases which relates the motion of the constituent atoms to the volume, pressure and temperature of the gas. The following topics will be covered:
((Ludwig Boltzmann)) Ludwig Eduard Boltzmann (February 20, 1844 – September 5, 1906) was an Austrian physicist famous for his founding contributions in the fields of statistical mechanics and statistical thermodynamics. He was one of the most important advocates for atomic theory when that scientific model was still highly controversial.
Boltzmann’s equation-curved in stone.
This equation was originally formulated by Ludwig Boltzmann between 1872 to 1875, but later put into its current form by Max Planck in about 1900. To quote Planck, "the logarithmic connection between entropy and probability was first stated by L. Boltzmann in his kinetic theory of gases."
Reference Carlo Cercignani, Ludwig Boltzmann The man who trusted atoms (Oxford University Press, 1998) E.A. Guggenheim, Boltzmann’s distribution law (North-Holland, 1955). G. Gallavotti, W.L. Reiter, and J. Yngvason edited, Boltzmann’s Legacy (European Mathematical Society, 2008).
1 Molecules colliding with wall
We consider molecules that strike a unit area of the wall of a container. Let vx denote the velocity component normal to the plane of the wall.
The particles are inside the cube with the volume V. There are N particles. We
consider the particle (mass m) with the velocity (vx ,vy ,vz ) . After the reflection at the wall,
the velocity of the particles is changed into (vx ,vy ,vz ) .The change of the linear momentum gives a force on the plane normal to the x axis.
px 2mvx F1xt
As a result of the Newton’s third law (the action-reaction), the force
2mv F F x x 1x t is exerted on the wall (as an impulse), contributing to the pressure. Here we assume that the number of molecules (per unit volume), which have velocity componenets of vx – (vx+dvx), vy – (vy+dvy), and vz – (vz+dvz) is given by
1 1 N (v)d 3v N (v ,v ,v )dv dv dv V V x y z x y z
3 where N(v)d v is the number of molecules whose velocities are vx – (vx+dvx), vy –
(vy+dvy), and vz – (vz+dvz.).
N N (v)d 3v
The number of particles (velocity v) colliding with the plane is evaluated as follows.
The volume of the cylinder is given by
Avxt
The number of molecules in the cylinder is calculated as
1 (Av t) N(v)d 3v x V
The force applied along the x axis is
2mv 1 A (Av t) x N(v)d 3v 2mv 2 N(v)d 3v x t V V x
Then the pressure P is defined by
F total 1 2Amv 2 P x x N(v)d 3v A A V
1 2mv 2 N(v)d 3v V x vx 0 where only the particles with vx>0 contribute to the pressure.
2 Thermal equilibrium In thermal equilibrium, the gas is assumed to be homogeneous for each direction. So it is natural to consider that N(v) is dependent only on the magnitude of v.
1 1 PV 2mv 2 N(v)d 3v 2mv 2 N(v)d 3v mv2 N(v)d 3v x 2 x 3 vx 0
2 2 where vx is replaced by v /3. Here we note that average of the total energy is
1 E mv2 N(v)d 3v 2
Then we have
2 PV E . 3
This equation is called a Bernoulli’s equation.
For an ideal gas, we have the Boyle’s law
PV NkBT , leading to the expression for E
3 E N Nk T 2 B
Note that is the average energy of each particle.
3 k T 2 B
3 Maxwell velocity distribution f(v)
3.1 Determination of the form N(v) It is assumed that N(v) depends only on the magnitude of v. The velocity distribution for vx is independent of those for vy and vz. Then we have the form of N(v) as
2 N(v) G(v ) g(vx )g(vy )g(vz ) (1) with g(0) = , where G and g are functions to be determined. In Eq.(1), we put vy = vz = 0.
2 2 G(vx ) g(vx )g(0)g(0) g(vx )
Then we have
1 g(v ) G(v 2 ) x 2 x
Similarly,
1 g(v ) G(v 2 ) y 2 y
1 g(v ) G(v 2 ) z 2 z
Then Eq.(1) can be rewritten as
2 2 2 2 G(v ) G(vx vy vz ) g(vx )g(vy )g(vz ) 1 G(v 2 )G(v 2 )G(v 2 ) 6 x y z
2 2 2 For simplicity we put vx = , vy = , andvz = . Then we have
1 G( ) G( )G()G( ) . 6
Taking a derivative of the above equation with respect to and putting = = 0,
1 G'( ) G( )G'(0)G(0) aG( ) 6 or
G( ) Aea where
1 a G'(0)G(0) . 6
The final form of N(v) is obtained as
2 2 2 2 2 2 2 N(v) G(vx v y vz ) Aexp[a(vx v y vz )] Aexp(av )
(Gaussian form)
Now we determine the values of A and a.
(a) Determination of A
N N(v)d 3v Aexp[a(v 2 v 2 v 2 )]dv dv dv x y z x y z
A[ exp(av 2 )dv ]3 x x A( )3/ 2 a or
A ( )3 / 2 N a where
exp(av 2 )dv exp(av 2 )dv x x x x a
((Note)) We can also calculate the above integral as follows.
N A 4v2 exp(av2 )dv 0 A 4 v2 exp(av2 )dv 2 1 2A a3 / 2 2 A( )3 / 2 a
((Mathematica)) J n_ : x n Exp a x2 x Simplify , a 0 &
list1 Table n,J n , n,0,10,2 ; list1 TableForm
0 a 2 2a3 2 3 4 5 2 4a 15 6 7 2 8a 105 8 9 2 16 a 945 10 11 2 32 a
(b) Determination of a The total energy
1 E m(v 2 v 2 v 2 )N(v)dv dv dv 2 x y z x y z 1 m(v 2 v 2 v 2 )Aexp[a(v 2 v 2 v 2 )]dv dv dv 2 x y z x y z x y z 1 mv2 Aexp(av2 )4v2dv 2 1 mA4 v4 exp(av2 )dv 2 0 1 1 mA4 v4 exp(av2 )dv 2 2 1 1 3 mA4 2 2 4a5/ 2 3 3/ 2 mA 4a5/ 2
2 where v is the radius of the sphere in the (vx, vy, vz) and 4v dv is the volume element enclosed by the spherical shell of inner radius v and outer radius v +dv,
v2 v 2 v 2 v 2 x y z 3 2 d v dvxdvy dvz 4v dv
Then we have
3 3/ 2 3 mA m E 5/ 2 5/ 2 3m 3 4a 4a k T 1 B N A( )3/ 2 4a 2 a a3/ 2
From this equation, the value of a is determined as
m a 2kBT
In summary we have the Maxwell distribution or Maxwell-Boltzmann distribution,
m mv 2 N(v) N( )3 / 2 exp( ) . 2kBT 2kBT
We define n(v) as
N(v) m mv 2 n(v) ( )3/ 2 exp( ) N 2kBT 2kBT with
1 n(v)d 3v n(v)4v2dv
(1) n(v)dv is the probability of finding particles whose velocities are vx – (vx+dvx), vy – (vy+dvy), and vz – (vz+dvz.).
(2) Note that n(v)4v2dv is the probability of finding particles whose magnitude of velocities are between v and (v + dv).
Using the Avogadro number NA, where
NAm = M (the molar mass) NAkB = R (gas constant), n(v) can be rewritten as
3/ 2 M Mv2 n(v) exp( ) 2RT 2RT
The function n(v) has a Gaussian type with a peak at v = 0. Here we introduce the probability f (v) dv of finding the particle having the velocity between v and v + dv, as
f (v)dv 4v 2n(v)dv
3/ 2 2 m 2 mv 4v exp( )dv 2k BT 2k BT 3/ 2 M Mv 2 4v 2 exp( )dv 2RT 2RT where
f (v)dv 1 0 f(v) is called the Maxwell velocity distribution and is given by
3/ 2 M Mv2 f (v) 4v2 exp( ) 2RT 2RT
((Note))
1 N(v)dv n(v)dv 4v2n(v)dv f (v)dv N
We note that f (v) has a local maximum at
2k T 2RT RT v B 1.41421 (most probable speed) mp m M M
((Note))
3/ 2 df (v) M Mv2 Mv2 Mv 4{2v exp( ) v2 exp( )( ) dv 2RT 2RT 2RT RT
3/ 2 M Mv2 Mv2 4 ( )v( 2) 0 2RT 2RT RT
The maximum value of f (v ) at v = vmp is
2 2M f max e RT
The distribution function is normalized as
f (v) v v ( )2 exp[1 ( )2 ] fmax vmp vmp
f fmax 1.0
0.8
0.6
0.4
0.2
v 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 vmp
Fig. Plot of the normalized f (v) / fmax as a function of a normalized v/vmp. Green: most probable speed (vmp) Blue: averaged speed (vavg) Brown: root-mean squared speed (vrms)
v v 6 1 avg ( 1.128) rms ( 1.225) vmp vmp 2
3.2 Calculation of average
v n v n f ( v ) d v
(a) The average speed vavg
2RT RT v v 2 1.59577 avg M M
(b) The root-mean squared speed vrms
3RT v 2 v2 rms M or
3RT RT v v2 1.73205 rms M M
((Note)) The definition of vrms 2 vrms is defined by
2 2 vrms (v v ) (v v ) v v v where v 0
(c)
2 RT RT v3 8 ( )3 / 2 6.38308( )3 / 2 M M
(d)
RT v4 15( )2 M
In summary,
vmp vavg vrms or
v v 6 1 avg ( 1.128) rms ( 1.225) vmp vmp 2
3.3 Calculation by Mathematica
3.4 Molecular speed
((Molecular velocities)) As is shown above, the most probable speed is given by
RT v 1.41421 mp M
From this form, we get the relation
M vmp 2 RT where
R = 8.314472 J/mol K
0 When M (kg/mol) = M0 (g/mol), and vmp (m/s) = vmp (0.1 km/s),
-3 2 0 M = 10 M0, vmp = 10 vmp .
Then we have
3 2 0 10 M 0 10 vmp 2 RT
0 2 1 M 0 vmp RT RT 102 103 5
6 Since v 0 v 0 , we have rms 2 mp
6 M v 0 RT 0 rms 2 5 which is equal to 26.095 at T = 273 K.
______Table The root-mean square speed for various gases (experimental values at T = 273 K) ______
0 0 0 vmp (0.1 km/s) vavg (0.1 km/s) M0 (g/mol) vrms M 0
H2 18.4 16.9 2.0158 26.0 He 13.1 12.1 4.003 26.2 H2O 6.2 5.7 18 26.30 Ne 5.8 5.3 20.18 26.05 N2 4.9 4.5 28 25.93 O2 4.6 4.2 32 26.02 Ar 4.3 4.0 39.95 27.18 Kr 2.86 2.63 83.9 26.20 Xe 2.27 2.09 131.3 26,0
______
((Example)) Calculation of vmp, vavg, and vrms
H2: M = 2.0 g/mol = 2.0 x 10-3 kg/mol T = 300 K
3 vmp = 1.579 x 10 m/s 3 vavg 1.782 x 10 m/s 3 vrms = 1.934 x 10 m/s
3.5 Zartman-Ko experiment A beam of molecules that emerged from a slit in the wall of an oven provided that basis of a direct test of the Maxwell-Boltzmann velocity distribution. This experiment, performed in the early 1930s by I.F. Zartman and C.C. Ko, made use of the apparatus shown in Fig. The oven contains Bi (bismuth) vapor at about 800°C, some of which escapes through a slit and is collimated by another slit a short distance away. Above the second slit is a drum that rotates about a horizontal axis at 6,000 rpm. At those instants when the slit in the drum faces the Bi beam, a burst of molecules centers the drum. These molecules reach the opposite face of the drum, where a glass plate is attached, at various times, depending upon their speeds. Because the drum is turning, the faster and slower molecules strike different parts of the plate. From the resulting distribution of deposited Bi on the plate it is possible to infer the distribution of speeds in the beam, and this distribution agrees with the prediction of Maxwell-Boltzmann statistics.
3.6 Neutron diffraction 3.6.1 Moderator Nuclear reactors provide a copious source of thermal neutrons. To a reasonable approximation, neutrons produced by nuclear fission are moderated within the reactor to form a gas with a Maxell-Boltzmann distribution of speeds corresponding to a temperature equal to that of the moderating material. This moderating material is typically light or heavy water at a temperature somewhat above 300 K, though if more or less energetic neutrons are required it is possible to locally heat or cool a part of the moderator and so produce a hot source or a cold source. Figure plots the Maxwellian flux distribution coming from moderators at 320 K and at 25 K. The peak flux for a temperature of 320 K occurs at a wavenumber of 4.5 Å-1, which corresponds to a neutron wavelength of 1.40 Å. By cooling the moderator to 25 K the peak flux is shifted to a wavenumber of 1.25 Å-1, corresponding to a neutron wavelength of 5.0 Å. These wavelengths are of the order of interatomic distances, so that neutrons are well suited to probe properties on an atomic length scale.
Fig. Flux distribution in the beam of neutrons from a moderator at 25 K and from a moderator at 320 K. The distributions are normalized to have the same total flux (M.F. Collins, Magnetic Critical Scattering, Oxford University Press 1989).
3.6.2 Neutron temperature In the thermal neutron energy range, the velocity distribution of neutrons follows the Maxwell-Boltzmann distribution according to which the most probable speed of the neutrons is:
1 eV = 1.602176487 x 10-19 J -27 mn = 1.674927211 x 10 kg (mass of neutron) 23 NA = 6.02214179 x 10 -23 kB = 1.380650410 x 10 J/mol K (Boltzmann constant) R = 8.314472 J/K (gas constant) -3 Mn=mnNA = 1.00728 x 10 kg (molar mass of neutrons)
At room temperature (20 °C = 293 K)
vmp = 2199.34 m/s vavg = 2481.69 m/s Emp = 25.25 meV. Eavg = 27.4299 meV
At 25 K (liquid hydrogen temperature)
vmp = 642.433 m/s vavg = 724.908 m/s Emp = 2.15434 meV. Eavg = 2.74299 meV
At 4.2 K (liquid helium temperature)
vmp = 263.319 m/s vavg = 297.124 m/s Emp = 0.361928 meV Eavg = 0.460822 meV
The neutron temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in eV. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adopted to the Maxwell-Boltzmann distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy is of the free neutron. Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation.
Fast neutrons have an energy greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition. Slow neutrons have an energy less than or equal 0.4 eV. Epithermal neutrons have an energy from 0.025 to 1 eV. Hot neutrons have an energy of about .2 eV. Thermal neutrons have an energy of about 0.025 eV. Cold neutrons have an energy from 5x10-5 eV to 0.025 eV. Very cold neutrons have an energy from 3x10-7 eV to 5x10-5 eV. Ultra cold neutrons have an energy less than 3x10-7 eV.
The wanelength of the neutron is given by
h 30.7895 n Å mnvmp T (K)
1.79874 Åat 293 K 6.15789 Å at 25 K 15.0237 Å at 4.2 K
4 Equipartition of energy 3 1 The energy k T is ascribed to a contribution k T from each “degree of 2 B 2 B freedom” of each particle, where the number of degrees of freedom is the number of dimensions of the space in which the atoms move: 3 in this example.
1 2 1 2 1 3kBT 3 mvrms mv m kBT 2 2 2 m 2 2 2 2 2 2 v vx vy vz 3 vx or
1 1 mv 2 k T 2 x 2 B
1 2 1 mvy kBT 2 2 1 1 mv 2 k T 2 z 2 B 1 3 m(v 2 v 2 v 2 k T 2 x y z 2 B
Let us now modify one of the basic assumption in the model of the ideal gas. Instead of considering a molecule to be presented as a point particle, let it be considered as two point particles separated by a given distance. This model gives a better description of diatomic gases. Such molecules can acquire kinetic energy by rotating about its center of mass, and it is therefore necessary to consider in the internal energy the contribution of rotational kinetic energy as well as the translational kinetic energy. The rotational kinetic energy of a diatomic molecules can be written as
1 1 K I 2 I 2 rot 2 x' x' 2 y' y' where I is the rotational inertia of the molecule for rotations about a particular axis. The x’y’z’ coordinate system is fixed to the center of mass of the molecule. There is no kinetic energy associated with rotation about the z’ axis, because Iz’ = 0.
1 2 1 I x'x' kBT 2 2 1 1 I 2 k T 2 y' y' 2 B
5 Heat capacity of the system with the degree of freedom (f) The specific heat of the gas is best conceptualized in terms of the degrees of freedom of an individual molecule. The different degrees of freedom correspond to the different ways in which the molecule may store energy. If the molecule could be entirely described using classical mechanics, then we could use the theorem of equipartition of energy to predict that each degree of freedom would have an average energy in the amount of (1/2)kT, where kB is Boltzmann’s constant and T is the temperature. Our calculation of the heat content would be straightforward. Each molecule would be holding, on average, an energy of (f/2)kBT where f is the total number of degrees of freedom in the molecule. The total internal energy of the gas would be (f/2)NAkBT, where NA is the total number of molecules. The heat capacity (at constant volume) would then be a constant (f/2)NAkB, the specific heat capacity would be (f/2)kB and the dimensionless heat capacity would be just f/2.
6 The case for diatomic molecules With complex molecules, other contributions to internal energy must be taken into account. The number of freedom is f = 7. E = f RT. CV = fR = 29.1 J / mol.K and γ= 1.29 (a) One possible energy is the translational motion of the center of mass. The three degrees of freedom described above are associated with the translational motion of the molecules in 3 independent directions. (3)
(b) Rotational motion about the various axes We can neglect the rotation around the y axis since it is negligible compared to the x and z axes. Rotational motion: 2 degrees of freedom (z and x). Rotational kinetic energy 1 1 I 2 I 2 . (2) 2 z 2 x
(c) Vibration of diatomic molecules The molecule can also vibrate. There is kinetic energy and potential energy associated with the vibrations. This adds two more degrees of freedom. (2)
1 E K U 2 K 2 U k T 2( k T) B 2 B
7 Agreement with Experiment At low temperatures, a diatomic gas acts like a monatomic gas CV= 3/2 R. At about room temperature, the value increases to CV= 5/2 R. This is consistent with adding rotational energy but not vibrational energy. At high temperatures, the value increases to CV= 7/2 R. This includes vibrational energy as well as rotational and translational
8. Mean free path l 8.1 Definition The mean free path or average distance between collisions for a gas molecule may be estimated as follows. If the molecules have diameter d, then the effective cross-section for collision can be modeled by
using a circle of diameter 2d to represent a molecule's effective collision area while treating the "target" molecules as point masses. In time t, the circle would sweep out the volume shown and the number of collisions can be estimated from the number of gas molecules that were in that volume.
The mean free path (the mean distance per collision) could then be taken as the length of the path divided by the number of collisions.
vt 1 l 2 2 (d vt)nV d nV
2 Where d vt is the volume of interaction, nV is the number of molecules per unit volume. The problem with this expression is that the average molecular velocity is used, but the target molecules are also moving. The frequency of collisions depends upon the average relative velocity of the randomly moving molecules.
8.2 Refinement of mean free path The intuitive development of the mean free path expression suffers from a significant flaw - it assumes that the "target" molecules are at rest when in fact they have a high average velocity. What is needed is the average relative velocity, and the calculation of that velocity from the molecular speed distribution yields the result.
2 2 2 2 v1 v2 v1 v2 2 v1 v2 2v since v1 = v2 = v and
v1 v2 v1v2 cos 0
Then the average relative velocity is given by
2 vrel v1 v2 2v
which revises the expression for the effective volume swept out in time t. The resulting mean free path is
1 l 2 2d nV
The number of molecules per unit volume can be determined from Avogadro's number and the ideal gas law, leading to
N P N AP nV , V kBT RT where
PV = NkBT
Then the mean free path is obtained as
1 RT l 2 2 2d nv 2d N AP
The mean free path depends on T, P, and the diameter of gas atoms.
((Example)) Evaluation of mean free path (He atom) d = 2.2 Å (atomic diameter) The collision cross section = d2 = 15.2 x 10-20 m2 nV (the concentration of molecules of an ideal gas at 273 K and 1 atm, which is called a Loschmidt number)
N P n A 2.6867774 x 1025 atoms/m3 V RT
1 -7 l 2 = 2.44 x 10 m = 2440 Å d nv
The rate of collision:
v 1310 rms 5.37109 / s l 2.44107 where vrms = 1310 m/s for He gas.
((Note)) Being a measure of number density, the Loschmidt constant is used to define the amagat, a practical unit of number density for gases and other substances:
25 −3 1 amagat = nV = 2.6867774×10 m , such that the Loschmidt constant is exactly 1 amagat.
8.3 Estimation of the mean free path at high vacuum
Estimate the mean free path of an air molecule at 273 K and 1 atm, assuming it to be a sphere of diameter 4.0 x 10-10 m (4Å). Estimate the mean time between collisions for an oxygen molecule under these conditions, using v = vrms = 517 m/s
RT (8.31J / K)(273K) l 5.2342108 m 2d 2 N P 2 (41010 )2 (6.021023 )(1.01105 Pa) A 5.2342108 m t 1.01010 s 517m / s
How about the mean free path at the high vacuum?
P = 10-n Torr = 10-n mmHg = (10-n)/760 atm = (10-n)/760 x (1.01x105) Pa
RT (8.31J / K)(273K) l 2d 2 N P 10n A 2 (410 10)(6.021023 )(1.01105 Pa) 760 76010n 5.2342108 m 3.97810n5 m
For P = 10-5 Torr (in the laboratory), we have
l = 3.978 m.
For P = 10-9 Torr (ultrahigh vacuum), we have
l = 3978 m.
((Link))
Mean free path calculation http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html
9. Solution of SP-19 and Hint of HW-19.
9.1
Problem 19-17***(SP-19) (10-th edition)
Container A in Fig. holds an ideal gas at a pressure of 5.0 x 105 Pa and a temperature of 300 K. It is connected by a thin tube (and a closed valve) to container B, with four times the volume of A. Container B holds the same ideal gas at a pressure of 1.0 x 105 Pa and a temperature of 400 K. The valve is opened to allow the pressures to equalize, but the temperature of each container is maintained. What then is the pressure in the two containers?
((Solution)) Before opening valve 5 PA = 5.0 x 10 Pa, TA = 300 K 5 PB = 1.0 x 10 Pa, TB = 400 K, VB = 4 VA
After opening valve, PA→P, TA = 300 K
PB→P, TB = 400 K.
Before opening the valve, P V n A A P V n RT A RT A A A A a PBVB nB RTB PBVB nB RTB
After opening the valve
PV n A PV n RT 1 RT A 1 A a PVB n2RTB PVB n2 RTB
The total number of moles remains unchanged.
PAV A PBV B 1 PAV A PBV B n A nB ( ) RT A RT B R Ta TB
PV A PV B P V A V B n1 n2 ( ) RT a RT B R Ta TB
Since nA nB n1 n2 , we have
P V T P V T P A A B B B A 2.0105 Pa VATB VBTA ______9.2
Problem 19-62***(SP-19) (10-th edition)
An ideal diatomic gas, with rotation but no oscillation, undergoes an adiabatic compression. Its initial pressure and volume are 1.20 atm and 0.200 m3. Its final pressure is 2.40 atm. How much work is done by the gas?
((Solution)) CP = 7R/2 CV = 5R/2
CP /CV 7 /5 1.40
Pf = 2.40 atm. Pi = 1.20 atm 3 Vi = 0.2 m Adiabatic process
PV PiVi Pf V f
1/ V P f i 21/ Vi Pf 1/1.40 3 V f 2 Vi 0.122m
The work done on the system is
V f V f W PdV PV V dV i i Vi Vi PV i i [V 1 V 1 ] 1 f i
Pf V f 1 PiVi 1 V f Vi 1 1 P V PV f f i i 1 1 1 (P V PV ) 1 f f i i 1.3312104 J
Then the work done by the system is + 1.2156 x 104 J. ______9.3
Problem 19-63***(SP-19) (10-th edition)
Figure shows a cycle undergone by 1.00 mol of an ideal monatomic gas. The temperatures are T1 = 300 K, T2 = 600 K, and T3 = 455 K. For 1 → 2, what are (a) heat Q, (b) the change in internal energy Eint, and (c) the work done W? For 2 → 3, what are (j) Q, (e) Eint, and (f) W? For 3 → 1, what are (g) Q, (h) Eint,, and (i) W? For full cycle, what are (j) Q, (k) Eint, and (j) W? The initial pressure at point 1 is 1.00 atm (= 1.013 x 105 Pa). What are the (m) volume and (n) pressure at point 2 and the (o) volume and (p) pressure at point 3?
((Solution)) n = 1 mol (monatomic gas) State-1: T1 = 300 K State-2 T2 = 600 K State-3 T3 = 455 K CV = 3R/2 = 12.47 J, CP = CV + R = 20.78 J.
Path 1-2 (a), (b), and (c)
Q CV T 12.47 300 3741.5J W 0 E Q W 3741.5J
Path 2-3 (adiabatic) (d), (e), and (f)
Q 0
E CV T 12.47(455 600) 1808.2J W E Q 1808.2J
Path 3-1 (g), (h), and (i)
Q CPT 20.78 (300 455) 3220.9J
E CV T 12.47 (300 455) 1932.9J W E Q 1288J
(j)
Qtotal 3742 0 3220 520J
(k)
Etotal 3742 1808 1933 0
(l)
Wtotal 0 1808 1288 520J
(m)
RT1 8.31447300 2 3 V1 5 2.4610 m P1 1.0132510
(n)
RT2 8.31447 600 5 P2 2 2.02810 2atm V2 2.4610
9.4
Problem 19-11** (HW-19) (10-th edition)
Air that initially occupies 0.140 m3 at a gauge pressure of 103.0 kPa is expanded isothermally to a pressure of 101.3 kPa and then cooled at constant pressure until it reaches its initial volume. Compute the work done by the air. (Gauge pressure is the difference between the actual pressure and atmospheric pressure.)
((My solution)) 1 atm = 1.01325 x 105 Pa 3 Vi = 0.140 m , Pi = 103.0+101.325 = 204.325 kPa Pf = 101.3 kPa
Path 1-2 (isothermal process)
PiVi Pf V f
V f W12 PdV V i
Path 2-3 (isobaric process)
W23 Pf (Vi V f )
______9.5
Problem 19-23** (SP-19) (10-th edition)
A beam of hydrogen molecules (H2) is directed toward a wall, at an angle of 55ºwith the normal to the wall. Each molecule in the beam has a speed of 1.0 km/s and a mass of 3.3 x 10-24 g. The beam strikes the wall over an area of 2.0 cm2, at the rate of 1023 molecules per second. What is the beam’s pressure on the wall?
A = 2.0 cm2 = 2.0 x 10-4 m2 -27 23 3 m = 3.3 x 10 kg, N0 = 10 moles/s, v = 1.0 x 10 m/s
N0t is the number of particles striking the wall over an area during the time t
The change of the linear momentum along the z axis,
pz N0t(2mvcos ) Fzt
______9.6
Problem 19-37** (SP-19) (10-th edition)
Figure shows a hypothetical speed distribution for a sample of N gas particles (note that
P(v) 0 for v>2v0. What are the values of (a) av0, (b) vavg/v0, and (c) vrms/v0? (d) What fraction of the particles has a speed between 1.5 v0 and 2.0 v0?
1 P(v)dv v v vP(v)dv avg 2 vrms v 3 v2 v2P(v)dv v v2 rms v0 v0 v Pr obability f P(v)dv vi
______9.7
Problem 19-57** (SP-19) (10-th edition)
The volume of an ideal gas is adiabatically reduced from 200 L to 74.3 L. The initial pressure and temperature are 1.00 atm and 300 K. The final pressure is 4.00 atm. (a) Is the gas monatomic, diatomic, or polyatomic? (b) What is the final temperature? (c) How many moles are in the gas?
((Solution))
Ti = 300 K, Vi = 200 L, Vf = 74.3 L Pi = 1 atm, Pf = 4.0 atm 1 L= 1000 cm3 = 10-3 m3
For the adiabatic process, we have the relation
PiVi Pf V f P V f ( i ) Pi V f C 2 p 1 Cv f P V PV f f i i nR Tf Ti
______10.5
Problem 19-70 (SP-19) (10-th edition)
3 An ideal gas, at initial pressure T1 and initial volume 2.0 m , is expanded adiabatically to a volume of 4.0 m3, then expanded isothermally to a volume of 10 m3, and then compressed adiabatically back to T1. What is its final volume?
3 3 3 V1 = 2.0 m , V2 = 4.0 m , V3 = 10.0 m
Adiabatic process (path 1-2) P1V1 P2V2 PV PV , 1 1 2 2 T1 T2
Isothermal process (path 2-3)
T T 2 3 P2V2 P3V3
Adiabatic process (path 3-4)
P3V3 P4V4 PV PV 3 3 3 3 T3 T4
Isothermal process (path 4-1)
T T 4 1 P4V4 P1V1
______10.6
Problem 19-81 (HW-19) (10-th edition)
An ideal gas is taken through a complete cycle in three steps; adiabatic expansion with work equal to 125 J, isothermal contraction at 325 K, and increases in pressure at constant volume. (a) Draw a P-V diagram for the three steps. (b) How much energy is transferred as heat in step 3, and (c) is it transferred to or from the gas?
Adiabatic process (path-1)
Q1 0 Isothermal process (path-2)
E2 0 Isobaric process (path-3)
W3 0
E 0 E1 E2 E3 (Joule’s law)
11. Link
Mean free path http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html
Lecture note (University of Rochester) http://teacher.pas.rochester.edu/phy121/LectureNotes/Chapter18/Chapter18.html
Kinetic theory http://en.wikipedia.org/wiki/Kinetic_theory
Maxwell-Boltzmann distribution http://en.wikipedia.org/wiki/Maxwell-Boltzmann_distribution
Molecular speed http://hyperphysics.phy-astr.gsu.edu/Hbase/kinetic/kintem.html